Scientists have observed a gigantic gas halo stretching from Andromeda – our nearest major galaxy. It goes towards our own Milky Way and if it could be seen with the naked eye, it would be the diameter of 100 full moons.

Halos are the gaseous atmospheres of galaxies. Astrophysicist
Nicolas Lehner of the University of Notre Dame, Indiana, who led
the study, explains that the properties of these gaseous
halos control the rate at which stars form in galaxies.

Andromeda, also known as Messier 21, or M31, is our closest major
galactic neighbor, at 2.5 million light-years away. It is also 25
percent more luminous than the Milky Way and contains about 1
trillion stars, twice as much as the greater estimates for our
own galaxy.

Both Andromeda and the Milky Way are part of a cluster called the
Local Group, containing about 45 other major galaxies that we
know of.

The scientists said that if the halo could be viewed with the
naked eye, it would be 100 times the diameter of the full moon ,
while Andromeda galaxy looks about six times the diameter of the
full moon.

The halo which appeared to be about six times larger and 1,000
times more massive than previously thought, stretches about 1
million light years from Andromeda, halfway to the Milky Way.
Scientists say it contains as much mass in its diffuse gas as
half of the stars in the Andromeda Galaxy. But the gas is
invisible, that is unless you find a bright enough background
object in the universe whose light is affected by the intervening
gas in the halo. The most convenient one is a quasar, a star-like
object.

The quasar’s extreme luminosity allows researchers to measure the
difference in the intensity of its glow, much in the same way
scientists look for planets orbiting distant stars.

Once again, the Hubble telescope proved it was too early to plan
for retirement. As associate professor of physics at Notre Dame,
J. Christopher Howk, explained that as the light from the quasars
travels toward Hubble, “the halo’s gas will absorb some of that
light and make the quasar appear a little darker in just a very
small wavelength range. By measuring the dip in brightness, we
can tell how much halo gas from M31 there is between us and that
quasar.”

Hubble possesses a unique capability to study ultraviolet light
at very high spectroscopic resolutions. This is especially good
for halos, as it allows getting fundamental information about
their nature and extent. It’s just a matter of finding enough
sources of ultra-bright light.

Although scientists observed other halos in the past, this one is
by far the largest located so close to us. The team used data
from 18 different quasars located at varying distances.

Lehner said that this is “a new milestone, because,
typically, only one quasar is used to probe the halos of galaxies
beyond the Local Group” while in this case “we have
assembled a large sample of quasars that directly demonstrate the
true extent of the halo of a single massive galaxy."

The team pooled Hubble’s quasar data for the last five years,
combining it to arrive at a more complete picture of the halo. It
hopes in future to find more quasar data to get an even better
understanding of the halo’s relationship to its galaxy.

But there is also the exciting possibility that Milky Way might
have a halo of its own. In this case, our two galaxies’ halos
could merge way before the galaxies’ actual collision which is
expected to give birth to a new elliptical galaxy in about 4
billion years.